Traditionally flexible pipe is utilized to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater, say 1000 metres or more) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 metres. Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including metallic and polymer layers. A riser is an assembly of one or more segments of flexible pipe connecting a sub-sea source to a surface station or vessel. A jumper is an assembly of flexible pipe connecting a sub-sea location to a further sub-sea location.
In some flexible pipe designs the pipe body includes one or more tensile armour layers. The primary loading on such a layer is tension. In high pressure applications, such as in deep and ultra deep water environments, the tensile armour layer experiences high tension loads from a combination of the internal pressure end cap load and the self-supported weight of the flexible pipe. This can cause failure in the flexible pipe since such conditions are experienced over prolonged periods of time.
Unbonded flexible pipe has been used for deep water (less than 3,300 feet (1,005.84 metres)) and ultra deep water (greater than 3,300 feet) developments. It is the increasing demand for oil which is causing exploration to occur at greater and greater depths where environmental factors are more extreme. For example in such deep and ultra-deep water environments ocean floor temperature increases the risk of production fluids cooling to a temperature that may lead to pipe blockage. Increased depths also increase the pressure associated with the environment in which the flexible pipe must operate. As a result the need for high levels of performance from the tensile armour layer(s) of the flexible pipe body is increased.
To alleviate the above-mentioned problem of loading on the tensile armour layer, buoyancy aids may be added to the flexible pipe at predetermined locations along the length of a riser. For deep water applications, the buoyancy aids can provide an upward lift to counteract the weight of the riser, effectively taking a portion of the weight of the riser, at various points along its length.
Buoyancy aids can be formed as separate modules of highly buoyant material, or as hollow shells that can be filled with such buoyant material. Such buoyancy aids can be clamped or otherwise secured to desired positions on a flexible pipe.
Flexible pipe may also be used for shallow water applications (for example less than around 500 metres depth) or even for shore (overland) applications. Flexible pipe for shallow water applications may also employ buoyancy aids.
Each buoyancy aid is usually fitted to a flexible pipe at the point of use, i.e. when a pipe is payed out into the water from a floating facility, because flexible pipe generally cannot be stored on a reel when it includes bulky buoyancy aids. It would be useful to provide a flexible pipe that is less labor-intensive to put into use, whilst also being cost effective.
Another problem associated with riser assemblies is known as birdcaging, which is the lateral buckling of wires of a flexible pipe due to compressive forces acting on the pipe. This is more usually encountered at the touchdown area of a pipe (the point where a riser touches the sea bed for example) and may be caused by a vertical reciprocal movement of the riser due to wave action. Buoyancy aids attached to a riser may increase the ranges and severities of (repeated) compression and tension to the pipe in the touchdown area when a riser experiences vertical movements.